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The "Monode" Noise GeneratorApril
1967 QST Article

April 1967 QST

[Table
of Contents]These articles are scanned and OCRed from old editions of the
ARRL's QST magazine. Here is a list of the
QST articles I have already posted. All copyrights (if any) are hereby acknowledged.

Calibrated
noise diodes are fairly inexpensive these days and are widely used for
measuring noise figure of systems and for generating specific signal-to-noise
ratios when testing receiver performance. This article from a 1967 edition
of QST describes a method for using a 'hot resistor,' aka 'monode,'
as a noise reference source. When the temperature (T) and the resistance
(R) is known, a noise power can be calculated with a precision limited
by the precision of the T and R measurements. The tungsten filament
of a pilot lamp is used as the resistor.

The "Monode" Noise Generator

Hot-Resistor Noise-Figure Measurement

By Ronald E. Guentzler,
W8BBB

This article describes a noise generator that should find
use in amateur work either as a noise source for noise-figure measurements
or as a reference source for comparison with the output from some other
noise source. It is inexpensive and simple to construct. The "Monode"
noise generator is essentially a hot resistor whose noise output is
known when the temperature and resistance are known.1 The
hot resistor is the tungsten filament of a No. 12 radio pilot lamp heated
from a d.c. source. The term "Monode" is derived from vacuum-tube terminology,
a monode being a one-element vacuum tube.

The Monode noise generator
was constructed to obtain a known source of random noise to check the
performance at 147 Mc. of a 5722 temperature-limited diode generator
similar to the one in the Handbook.2 The reason for desiring
a means of checking the 5722 generator arose from comments by J. A.
Huie3 and A. van der Ziel4 regarding the effects
of stray capacitance and inductance on the noise output of the 5722
generator at high frequencies. (The output of a 5722 generator was found
to be 12 percent or 0.5 db. high at 147 Mc. before compensation!)

Two other Monode noise generators were built to prove that the
principle of the Monode noise generator was indeed practical at lower
frequencies. These generators are for use in the 6- and 40-meter bands.

The Resistor as a Noise Generator

A
resistor at any temperature above absolute zero generates a noise power

P = KTB watts, where

K = 1.38 X 10-23 Joules/Kelvin
degree, T is the temperature of the resistor in degrees Kelvin,
and B is the bandwidth in cycles per second.

When the temperature
of a resistor is other than some reference temperature, T0
(usually taken as 290°K), it may be convenient to use the terms
"excess noise temperature" or "excess temperature," which are defined
as the temperature of the resistor minus reference temperature; i.e.,

TE.N. = T - T0.

The term "excess
noise" is commonly used; the excess noise is the excess noise temperature
divided by the reference temperature; i.e.,

E.N. = (T - T0)
/T0.

The excess noise may be given in db., where
E.N.db. = 10 log10 (T - T0) /T0.

In order to obtain enough noise for convenience of measurement,
the resistor may be raised to many times room temperature. The filament
of an incandescent lamp makes a good hot resistor because tungsten is
a well-behaved material and has a high melting point. The temperature
can be raised by passing a direct current through it.

The Monode Generator

The Monode has the advantages
of simplicity, low cost, and being an absolute standard. The disadvantages
are fixed output and the necessity for tuning each amateur band (but
not within the band). The complete generator is composed of three basic
parts: a regulated variable-voltage power supply, a room-temperature
"quiet" termination (R1), and the noise generator with its
r.f. filtering and coupling network (Fig. 1).

A resistor is about as basic a noise generator as you can
get. The filament temperature of a No. 12 dial lamp can be adjusted
to the desired resistance with sufficiently high noise output,
and the corresponding noise temperature is available from the
calibration curve given in this article. With these data, measurement
of receiver noise figure becomes simple.

The variable d.c. voltage from the power supply is used to heat the
filament of the lamp. The d.c. is filtered by means of RFC1
and a 0.001-µ.f. capacitor, C3, to eliminate any r.f.
noise component that might be present in the power supply. RFC2
is used to conduct into the lamp the d.c. required to heat the filament
while preventing the thermal noise generated in the hot lamp filament
from being lost in the supply. The noise generated in the filament is
coupled to the output connector, J2, by means of C1;
this capacitor also serves the function of resonating the lead and lamp-filament
inductances so that the output impedance is purely resistive.

Fig. 1 - Circuit of the Monode noise
generator. Except as indicated, capacitances are in µf.; capacitor
with polarity marked is electrolytic, other fixed capacitors are disk
ceramic. Resistances are in ohms.

The major portion
of the noise generator can be built using any mechanical construction
desired. The one described was built on a 3 1/2 X 19-inch relay-rack
panel. The power supply is mounted in a 3 X 4 X 6-inch aluminum chassis
fastened to the rear of the panel. The r.f. filter network and the quiet
termination, R1, and its connector, J1, are mounted
in a small Minibox. The No. 12 lamp, C1, RFC2
and J2 are mounted in an identical Minibox. The two Miniboxes
are fastened together and to the panel; the connectors J1
and J2 protrude through holes in the panel.

One obvious
innovation would be to have the Minibox containing the lamp physically
separate from the power supply and connected to it by means of a flexible
cord. In this event, the coaxial socket J2 would be replaced
by a plug and R1 would be mounted in a separate plug.

The power supply is a conventional bridge-rectified, RC-filtered
supply with a shunt Zener regulator. The supply was made electrically
larger than necessary because it was not known at the time of construction
what lamp type would be used in the final version. The 1/2-ampere, 18-volt
capability gives a range of voltages and currents large enough for experimental
purposes. The actual maximum output required for the No. 12 lamp is
10 volts at 200 ma. The regulation is probably not necessary.

The noise-generator portion of the unit requires more than usual
care, considering the frequency for which the unit is designed. Stray
inductance and capacitance are not particularly important, although
they should be kept low; this is the opposite of the 5722 generator
where stray inductance and capacitance result in improper amounts of
noise output. However, losses cannot be tolerated; i.e., any resistance
appearing in the noise-generating circuit other than the hot lamp filament
must be eliminated. This is again the opposite situation from the 5722
generator where losses will, in general, have no deleterious effects
and can be beneficial.

Fig. 2 is a photograph of the noise-generating
portion of the unit. The components are mounted in such a way that the
lead lengths are as short as possible in order to keep their losses
low. The lamp is mounted in a "socket" constructed from two of the metal
inserts taken from a miniature-tube socket. An entire tube socket cannot
be used because the pin spacing is improper. Also, sockets introduce
the possibility of losses.

The Monode described here is usable
at frequencies below 144 Mc. with slight modification. Two separate
noise-generating portions were built, one for use on 6 meters and one
for use on 40 meters. For 6 meters, C1 is a 50-380-pf. mica
trimmer, RFC1 and RFC2 are Ohmite Z-50 inductors,
and C3 is the same as listed for 2 meters. For 40 meters,
C1 is two 0.001-µf. fixed mica capacitors in parallel,
RFC2 is an Ohmite Z-7 inductor, RFC1 is omitted,
and C3 is a 0.1-µf. ceramic.

For the other
high-frequency bands, use the appropriate Ohmite inductor for RFC2,
omitting RFC1; use a 0.1-µf. disk ceramic for C3.
C1 should be the size required to resonate the lamp filament
and lead inductance in order to present a pure 50 ohms at the output
connector.

Adjustment

Fig. 2 - The noise- generating head of the Monode. Leads
between the lamp, C1 and the coax connector are kept
to the shortest possible length. The quiet termination and r.f.
filter are in a similar box bolted to the bottom of the one
shown.

Although the temperature of the lamp filament can be varied by varying
the applied .c. voltage, only one temperature of operation is usable
because the resistance of the filament is also a function of the applied
voltage, and this resistance must be set to give the proper output impedance.
Some means of impedance measurement in the band in which the unit is
to be used should be available; this can be either an impedance bridge
or meter or an s.w.r. bridge known to be properly calibrated. The impedance-measuring
device must be sensitive enough to operate on small amounts of r.f.
This is necessary to insure that the r.f. getting into the lamp does
not heat the filament to a temperature greater than that resulting from
the applied d.c. A good check can be made by applying the r.f. while
the d.c. is off. The lamp should not glow.

With the impedance-measuring
device connected and operating, the d.c. lamp voltage is applied and
the voltage and C1 are adjusted until the output impedance
at connector J2 is 50 ohms, purely resistive. The value of
the lamp voltage is noted, and whenever the unit is to be used the voltage
is set at this value. If the Monode noise generator is to be used as
a reference for comparison with other noise generators, it is important
that the output impedances of all the generators be the same. The best
way to make sure that they are the same is to measure all of them at
the same time, with the same measuring device, and at the same frequency.

The operating temperature of the filament can be found from
Fig. 3. This curve applies to General Electric No. 12 lamps, and may
not be applicable to lamps of other than G.E. manufacture.5
The excess temperature or excess noise can be calculated by means of
previously given formulas. For example, assume that as a result of the
impedance adjustment step it was found that the lamp must operate at
8.4 volts in order to give an output impedance of 50 + j0. With the
aid of Fig. 3 the lamp temperature is found to be 2430°K when operated
at 8.4 volts. This is the noise temperature. If room, or reference,
temperature is 290°K, then the excess temperature is 2430 - 290
= 2140°K, and the excess noise in db. is 10 log10 (2140/290)
= 8.7 db.

R1 should be adjusted to give an impedance
of 50 + j0 when viewed through J2 This resistor should be
an inherently nonreactive composition type such as the Ohmite "Little
Devil."

Using the Monode Generator

In using the Monode as a source of noise for noise-figure measurements
of a receiver, the quiet termination of the Monode is first connected
to the input of the receiver under test by means of a coaxial cable
having x db. loss. (If the separate noise-head construction is used,
x is taken as zero.) The output noise power from the receiver is noted;
call this reading A. The Monode is then set to its operating voltage,
its output is connected to the input of the receiver through the same
cable, and the output power of the receiver is noted; call this reading
B. The noise figure of the receiver in db. is then found from the formula

N.F.db. = E.N.db. - x - 10 log10
(A-1), where E.N.db. is the excess noise of the
Monode noise generator in db. Note that B and A must be in units of
power and not in db.

For example, assume that a coaxial cable
with 0.9 db. loss is being used between the Monode noise generator and
the receiver; this makes x = 0.9 db. Assume that the excess noise of
the Monode is 8.7 db. (from the previous example). Further, assume that
the receiver noise output was 1 milliwatt with the quiet termination
and 4 milliwatts with the Monode connected; this makes B/A = 4. Therefore,
the receiver noise figure is 8.7 - 0.9 - 4.8 = 3.0 db.

Concluding Remarks

The following comments are
intended to provide a basis for further experimentation, especially
at frequencies above 150 Mc.

Skin effect does not significantly
increase the a.c. resistance of the lamp filament from the d.c. value,
at frequencies up to 150 Mc., because the filament diameter is small
(approximately 0.001 inch) and the resistivity of tungsten is relatively
high. Therefore, it would be expected that the resistive component of
the output impedance as seen from J2 would be the same as
the d.c. filament resistance as calculated by taking the ratio of the
lamp voltage and current. However, there is about 1 pf. shunt capacitance
across the lamp resulting from the lamp leads, and there is a significant
amount of inductance effectively in series with the filament resistance.
This inductance is principally a result of the coiled filament and the
filament support leads; the total inductance is approximately 0.04 microHenry.
As a result of the inductance and capacitance, the resistive portion
of the filament impedance, as viewed from J2, is increased
in magnitude. This increase is significant only at 50 Mc. and above.
For the unit pictured in Fig. 2, the d.c. lamp resistance is 45.6 ohms
when the output impedance is 50 + j0 at 147 Mc.

The impedance
step-up effect was not observed in an earlier version of this unit.
The d.c. filament resistance and the output impedance were both 50 ohms.
This was considered a bit of good luck until the noise output was found
to be too low. It was discovered that the inductor RFC2 was
lossy and, since it was effectively in parallel with the lamp, its associated
losses lowered the apparent filament resistance, the output impedance,
and the noise temperature. This is why the Ohmite inductors are specified.

A different lamp type might eliminate the impedance transformation
problem and the necessity for retuning or rebuilding the Monode noise
generator for each different amateur band. An ideal lamp type for noise
generation would be one with a straight (not coiled) filament mounted
in a small-diameter tubular bulb having the lead-in wires appearing
at the opposite ends of the lamp. If the leads are brought out from
opposite ends of the bulb, the lamp could be coaxially mounted. This
mounting scheme offers the possibility of low stray inductance and capacitance.

I wish to express my appreciation to Mr. Donn R. Hobbs of the
Miniature Lamp Department, General Electric Company, Nela Park for many
pieces of information regarding miniature lamps.